Apparatus and process for preparing semiconductor rods



Nov. 8, 1966 R. J. BINDER 3,

APPARATUS AND PROCESS FOR PREPARING SEMICONDUCTOR RODS Filed Oct. 15,1964 HIGH FREQUENCY 55 susCEPTOR GENERATOR T as 64 56 e|- j w HODRECTIFIER gfiI DETECTOR 2.] j g 0 0 II 0 x r L g c 0 o A II 0 SATURABLECURRENT SELF- CORE MAGNET'C ADJUSTING BALANCING REACTOR AMPL'F'ERTRANSMITTER POTENTIOMETER POWER sOuRC 5+ FIG-l INVENTOR RICHARD J.BINDER G3 GI PJJvZML-MT FIG.2 ATTORNEY United States Patent 3,284,172APPARATUS AND PROCESS FOR PREPARING SEMICUNDUCTOR RODS Richard J.Binder, St. Louis, Mo., assignor to Monsanto Company, St. Louis, Mo., acorporation of Delaware Filed Oct. 13, 1964, Ser. No. 403,489 6 Claims.(Cl. 23301) This invention relates to the preparation of semiconductorrods and is particularly concerned with an apparatus and process fortemperature regulation incident to the melt drawing of semiconductormaterials into monocrystalline semiconductor rods.

In conventional methods for the production of monocrystallinesemiconductor rods 'by the melt drawing of semiconductor materials, thesemiconductor material is melted within a susceptor by use of inductiveheating through the medium of a high frequency work coil and throughmanipulative arrangements a semiconductor rod is produced by the drawingor pulling of the melt into rod form. In melt drawing methods such asthe foregoing, it is of prime importance that the melt of semiconductormaterial 'be maintained at substantially optimum growing temperature. Itis well-known that in order to grow acceptable mono-crystallinesemiconductor rods, the delicate balance at the interface between themelt and the growing crystal must be maintained. For example, if thetemperature varies less than about 1 C. above or below the optimumgrowing temperature, such deleterious effects as crystal melting or meltfreezing will occur and such will affect the quality of the formedsemiconductor rod.

Various methods for controlling the temperature of the melt ofsemiconductor material during the melt drawing process are known andsuch methods rely on controlling the temperature by optical means,temperature-sensitive means, electrical means and various combinationsof these means. For example, in the automatic control bythermal-electrical means the temperature of the apparatus used incrystal pulling is controlled by a thermal-electrical system through thepower input to the high frequency source thereby controlling thetemperature of the melt of the semiconductor material. In this example,a secondary loop inductively coupled to the high frequency power circuitis used as a feed back loop to the primary electrical control system.However, such feedback means has many limitations among which are itstendency to be influenced by stray or spurious electromagnetic radiationfrequencies and its dependency on the space relationship of the feedbackloop to the work coil.

As will be more fully discussed hereinafter, this invention is directedto an improved electrical control means which minimizes or obviates thelimitations of the prior art methods.

According to the present invention, there is provided an improvedprocess and apparatus for the temperature regulation of the melt ofsemiconductor material as a monocrystalline semiconductor rod is pulledor drawn therefrom by means of a mechanical arrangement while thetemperature of the melt is controlled by primary control means whichsenses temperature fluctuations in the melt, converts the temperaturefluctuations to an error voltage representing the difference between thefluctuated temperature and a desired melt temperature and utilizes thiserror voltage to control the power of the work coil used as the heatingmeans to form the melt, and a secondary control circuit meanscapacitively coupled to the work coil which monitors the voltage of thework coil, is responsive to voltage changes in this coil and sums anyvoltage changes with the error voltage to stabilize the power in thework coil and thereby stabilize the temperature of the melt, all ofwhich will be more fully discussed hereinafter.

The various objects and features of the invention will be brought out inthe description and in order to facilitate the description andunderstanding of the invention, reference is made to the appendeddrawings in which:

FIGURE 1 shows the principles underlying the operation of one embodimentof the present invention; and

FIGURE 2 is a schematic diagram of a means for summing the voltagechange fro-m the cathode follower and the error voltage from theself-balancing potentiometer.

Reference is now made to FIGURE 1 which illustrates schematically anembodiment of the present invention. A semiconductor material 51, suchas silicon or germanium, is placed in a susceptor 52, such as a quartzlined graphite crucible, within an enclosure (not shown) which ismaintained at a temperature below the melting point of the semiconductormaterial in order to maintain the rod in a solid state as it is drawnfrom the melt. Suitable means are provided for supporting the susceptorwithin the enclosure. A high frequency work coil 53 connected to asuitable source of alternating voltage (high frequency generator 54) ismounted concentrically about a portion of the susceptor 52. Within thecircuit of the work coil 53 and the high frequency generator 54 aresonating capacitor 55 is provided in parallel to permit a largecirculating current in the resonant circuit of which the work coil is apart, while the high frequency source has to supply only a relativelysmall current. When the work coil is energized the semiconductormaterial within the susceptor is melted. The semiconductor material ispulled or drawn into rod form from the melt or puddle usually by meansof a seed crystal through a suitable holder and mechanical arrangement.

The primary control circuit for controlling the temperature of the meltof semiconductor material is provided by a thermal-electrical controlsystem. This system is provided with a detector 56, that is, athermopile or bolometer, which receives infra-red radiation from thesusceptor at the temperature at which such is operated. The intensity ofthis radiation is proportional to the temperature of the susceptor andthus provides a means through which changes in the temperature of thesusceptor can be detected which reflect temperature changes in the meltportion of the semiconductor material. The detector generates a smallpotential as a function of the incident radiation (infra-red radiation).This potential is impressed across a self-balancing potentiometer 57which is provided with a set point in order to provide a predeterminedpotential value and which measures the difference, if any, between theset point potential and the input potential and transmits thisdifference or error voltage to a current adjusting transmitter 58. Theself-balancing potentiometer can be provided, if desired, with arecording unit or visual indication unit which can record or indicatethe difference in potential. The current adjusting transmitter 58 isessentially a high gain and high impedance output amplifier and servesto generate a milliamp current from the millivolt potential differenceor error voltage input. The current output from the current adjustingtransmitter is applied to the magnetic amplifier 59 and amplifiedthereby. The output of the magnetic amplifier is applied to windingsformed on the core of a saturable core reactor 60. Thus, by varying thecurrent on the control winding, the reactor 60 can be employed tocontrol the power from a power source to the high frequency generator 54thereby controlling the power in the work coil and thus the temperatureof the susceptor and melt.

Provided, along with the primary control circuit, is a furtherelectrical control circuit (secondary control circuit) which improvesthe operation of the primary control circuit. The secondary controlcircuit is capacitively coupled with the high frequency power circuitand is responsive to changes in the voltage of the power circuit withoutmaterially influencing the work coil voltage and, when the voltage ofsuch circuit starts to change from a predetermined value, the change isimmediately detected :by the secondary control circuit which along withthe primary control circuit lends toward restoring the voltage to thepredetermined value thereby maintaining the melt with a more uniformtemperature and thus aiding in producing a semiconductor rod ofacceptable quality.

The secondary control circuit is provided with measuring capacitors, 61and 62, which prevent any appreciable current from being drawn from thehigh frequency power circuit, a rectifier 63 which rectifies thealternating current voltage to a pulsed direct current voltage, and acathode follower 64 which permits the pulsed direct current voltage fromthe circuit to be used and/ or measured without materially influencingthe voltage of the work coil circuit. The output from the cathodefollower (DC potential) is fed to the current adjusting transmitter 58where this potential i summed with the potential output from theself-balancing potentiometer 57, that is, the error voltage.

Referring now to FIGURE 2 which is a schematic diagram of a portion ofthe current adjusting transmitter which sums the voltage change from thecathode follower and the error voltage from the self-balancingpotentiometer. The current adjusting transmitter is provided with a highvoltage source B+(D.C. battery), a plate load resistor 70, an amplifyingtriode 71 and a cathode resistor 72 which are serially connected.Between the cathode resistor 72 and ground G there is provided inputconnections, 73 and 74, for impressing across the connections thevoltage change from the cathode follower (V f). The error voltage fromthe self-balancing potentiometer (V,,) is impressed across the grid 75of the amplifying triode 71 and ground G Between the plate load resistor70 and the amplifying triode 71 there is provided a connection 76between which and ground G the summation voltage (V,,) is taken to beused in the portion of the current adjusting transmitter which serves togenerate a millia-mp current from the summation voltage. As can beobserved from this figure, the summation voltage will at all times be afunction of the sum of the eflective grid voltage, that is, V +V Inoperation, the self-balancing potentiometer is set at a predeterminedpotential value so that no potential difference is generated therefromwhen the temperature of the susceptor and thus the melt is at itspredetermined temperature value. If, for any reason, the temperature ofthe susceptor starts to vary or fluctuate, this change is immediatelydetected via the detector and the selfabal-ancing potentiometer, whichas hereinbefore explained, generates a potential or error voltage usedto control, via the current adjusting transmitter, magnetic amplifierand saturable core reactor, the power source to restore the power in thework coil and thus the temperature of the susceptor to its predeterminedvalue. The error voltage from the self-balancing potentiometer,therefore, represents the difference between the fluctuated temperatureand a desired temperature. Such a control means, however, has manylimitations with perhaps the major limitation being the slowness ofresponse of the control circuit due to, among other things, the inherenttime lag between the change in power and the change in temperature ofthe melt which is dependent on the change in power, as well as the timedelay of the detector in generating the potential from the infra-redradiation received.

The secondary control circuit is provided in order to anticipate theprimary control circuit and improve its operating characteristics. Sincethe secondary control circuit monitors the voltage of the work coilwhich volttage is proportional to the temperature of the melt of thesemiconductor material, such circuit can detect changes in work coilvoltage from many sources, among which are variations in voltagesupplied to the power source, changes in load impedance of the work coilcircuit, and changes in the voltage due to changes in the power by theprimary control circuit in controlling the temperature of the melt.Additionally, the secondary control circuit is much more rapid inreacting to such changes in the voltage of the work coil circuit ascompared to the primary control circuit responding to temperaturechanges in the melt. Therefore, the secondary control circuit will tendtoward stabilizing the voltage of the work coil and thus the temperatureof the melt.

The secondary control circuit, being capacitively coupled with said workcoil, is provided wit-h adjusting means so that at a predeterminedvoltage in the work coil which corresponds to the desired temperature ofthe melt there is a fixed voltage output from the cathode follower. Inoperation, when the work coil voltage changes from its predeterminedvalue, the change is immediately detected "by the secondary controlcircuit, the potential output of which is fed to the current adjustingtransmitter and summed with the potential difierence or error voltage,if any, from the self-balancing potentiometer. Since, however, thechange, detected by the secondary control circuit, is transmitted to thecurrent adjusting transmitter more rapidly than any temperature changein the melt as reflected by the error voltage generated in theself-balancing potentiometer, the secondary control circuit tends toanticipate or override the primary control circuit and thus stabilizethe voltage of the work coil thereby tending to stabilize thetemperature of the melt of semiconductor material.

As can be appreciated from the foregoing, the electrical secondarycontrol circuit improves the operation of the melt drawing method andbecause it is capacitively coupled withinthe work coil circuit anaccurate and rapidly responsive control means is provided which resultsin the production of a mono-crystalline semiconductor rod of acceptablequality.

What'is claimed is:

1. In a method of melt drawing a monocrystalline semiconductor rodwherein a semiconductor material is melted within a susceptor by use ofinductive heating through the medium of a work coil energized by a highfrequency electrical source, and thereafter drawing said semiconductormaterial into rod form from said melt by means of a mechanicalarrangement while controlling the temperature of said melt by sensingtemperature fluctuations in said melt, converting said temperaturefluctuations to an error voltage representing the difference between thefluctuated temperature and a desired temperature, and utilizing saiderror voltage to control the power source to said high frequency energysource and thereby control the power in said work coil, the improvementwhich comprises utilizing changes in the voltage of said work coilwithout materially influencing said voltage 'by summing with said errorvoltage to stabilize the power in said work coil and thereby stabilizethe temperature of said melt.

2. In a method of melt drawing a monocrystalline semiconductor rodwherein a semiconductor material is melted within a susceptor by use ofinductive heating through the medium of a work coil energized by a highfrequency electrical source, and thereafter drawing said semiconductormaterial into rod form from said melt by means of a mechanicalarrangement while controlling the temperature of said 'melt by sensingtemperature fluctuations in said melt, converting said temperaturefluctuations to an error voltage representing the difference between thefluctuated temperature and a desired temperature, and utilizing saiderror voltage to control the power source to said high frequency energysource and thereby control the power in said work coil, the improvementwhich comprises monitoring the voltage of said work coil by means of acapacitively coupled electrical circuit, and summing changes in saidvoltage with said error voltage to stabilize the power in said work coiland thereby stabilize the temperature of said melt.

3. The method of claim 2, wherein said electrical circuit is comprisedof, in combination, measuring capacitors for preventing any appreciablecurrent from being drawn from said work coil, a rectifier for rectifyingthe alternating current votlage to a pulsed direct current voltage and acathode follower for permitting the pulsed direct current voltage to beused without materially influencing said work coil voltage.

4. In an apparatus for melt drawing a monocrystalline semiconductor rodcomprising a susceptor for containing a melt of semiconductor material,a work coil surrounding said susceptor for melting said semiconductormaterial within said susceptor by use of inductive heating when saidwork coil is energized by a high frequency electrical source, drawingmeans for drawing said semiconductor material into rod form from saidmelt, and temperature control means for controlling the temperature ofsaid melt comprising sensing means for'sensing temperature fluctuationsin said melt, means for converting said temperature fluctuations to anerror voltage representing the difference between the fluctuatedtemperature and a desired temperature, and means for utilizing saiderror voltage to control the power source to said high frequency energysource thereby controlling the power in said work coil, the improvementwhich comprises circuit means capacitively coupled to said work coil andresponsive to changes in the voltage of said work coil and means forsumming said voltage changes with said error voltage to stabilize thepower in said work coil and thereby stabilize the temperature of saidmelt.

5. Apparatus according to claim 4, wherein said circuit means iscomprised of, in combination, measuring capacitors for preventing anyappreciable current from being drawn from said work coil, a rectifierfor rectifying the alternating current voltage to a pulsed directcurrent voltage and a cathode follower for permitting the pulsed directcurrent voltage to be used without materially influencing said work coilvoltage.

6. Apparatus according to claim 5, wherein said means for summing saidvoltage changes with said error voltage is comprised of, in combination,a high voltage direct current voltage source, a plate load resistor, anamplifying triode and a cathode resistor for providing a summationvoltage which is a function of the effective grid voltage of saidtriode.

References Cited by the Examiner UNITED STATES PATENTS 2,752,473 6/ 1956Hoge 219-497 2,916,593 12/1959 Herrick 23-273 3,177,336 4/1965 Fischer219-497 3,206,286 9/1965 Bennett et a1. 23-301 NORMAN YUDKOFF, PrimaryExaminer. G. P. HINES, Assistant Examiner.

1. IN A METHOD OF MELT DRAWING A MONO-CRYSTALLINE SEMICONDUCTOR RODWHEREIN A SEMICONDUCTOR MATERIAL IS MELTED WITHIN A SUSCEPTOR BY USE OFINDUCTIVE HEATING THROUGH THE MEDIUM OF A WORK COIL ENERGIZED BY A HIGHFREQUENCY ELECTRICAL SOURCE, AND THEREAFTER DRAWING SAID SEMICONDUCTORMATERIAL INTO ROD FORM FROM SAID MELT BY MEANS OF A MECHANICALARRANGEMENT WHILE CONTROLLING THE TEMPERATURE OF SAID MELT BY SENSINGTEMPERATURE FLUCTUATIONS IN SAID MELT, CONVERTING SAID TEMPERATUREFLUCTUATIONS TO AN ERROR VOLTAGE REPRESENTING THE DFFERENCE BETWEEN THEFLUCTUATED TEMPERATURE AND A DESIRED TEMPERATURE, AND UTILIZING SAIDERROR VOLTAGE TO CONTROL THE POWER SOURCE TO SAID HIGH FREQUENCY ENERGYSOURCE AND THEREBY CONTROL THE POWER IN SAID WORK COIL, THE IMPROVEMENTWHICH COMPRISES UTILIZING CHANGES IN THE VOLTAGES OF SAID WORK COILWITHOUT MATERIALLY INFLUENCINNG SAID VOLTAGE BY SUMMING WITH SAID ERRORVOLTAGE TO STABILIZE THE POWER IN SAID WORK COIL AND THEREBY STABILIZETHE TEMPERATURE OF SAID MELT.